VIBRATIONAL SPECTROSCOPIC CHARACTERISTICS OF =C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O AND N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-PI INTERACTION IN CRYSTALLINE ,6-DIMETHYLPHENYL)-5-METHYLISOXAZOLE-3-CARBOXAMIDE

Infrared and Raman spectra have been scrutinised for features reflecti ng the presence of a very short =C-H ... O contact in crystalline ,6-d imethylphenyl)-5-methylisoxazole-3-carboxamide (D2624). A tentative as signment of infrared and Raman bands is presented for the pure and par tly deuterated compound measured in the solid state and in the non-pol ar solution. The intensity increase of the =C-4-H stretching vibration upon changing the matrix from a solution to the crystalline state, is the most prominent feature particularly in infrared. The temperature sensitivity was determined for the =C-4-H and N-H stretching vibration ; the wavenumber-temperature coefficients (WTc) are significant but sm all, 0,0137 and 0.0143 cm(-1) K-1, respectively. The presence of an N- H ...pi interaction in the crystalline phase is established by mimicki ng this type of interaction in solution. Infrared measurements at temp eratures above the melting point show that an intermolecular N-H ... O =C amide bond is present in the liquid phase. Upon cooling (crystallis ation) this typical energetically favourable amide bond is disrupted a s a result of several competing attractive forces (intermolecular =C-H ... O and N-H ...pi interaction, intramolecular N8-H ... N2) and the effect of steric shielding of the amide functionality by the 2,6-dimet hylphenyl group.